Since the inception of implantable cardiac devices, these devices have become increasingly sophisticated and more capable over time. The initial implantable cardiac devices were typically comprised of pacemakers which provided electrical pacing pulses to the heart at a generally fixed rate. As the technology has developed, more advanced pacing systems have been implanted in patients which, for example, are capable of providing pacing pulses to the heart only when the pacing system determines that the heart will not provide an intrinsic heart beat. Moreover, these advanced pacemakers are also able to adjust the pacing rate to accommodate different levels of physical activity and corresponding metabolic demand of the patient.
One continuing problem with pacing systems is that the pacing system must preferably be able to adjust the rate at which pacing pulses are delivered to the patient, such that the patient's heart rate more closely tracks the metabolic demand of the patient. In other words, as the patient needs more oxygenated blood to be carried to their extremities during heightened physical activity, the pacing rate of the pacemaker should be increased such that the heart is induced to beat faster to pump more oxygenated blood.
It is generally understood that, with demand-type pacing systems, the desired heart rate is the rate at which the heart must beat to meet the existing metabolic demand of the body. The pacing system typically does not provide pacing pulses at this rate, but simply ensures that either paced events or intrinsic heart events occur at the desired heart rate.
Typically, pacing systems are equipped with sensors which provide signals that are used by the control unit of the pacing system to determine the pacing rate. One such sensor is an activity sensor that typically includes an accelerometer that is positioned within the housing of the control unit that is implanted within the patient's body. As the patient becomes more active, the accelerometer measures the resulting acceleration and provides an activity signal that is indicative of the increased acceleration experienced by the patient. Activity sensors of this type are generally thought to provide a very good indication of the metabolic demand of the patient for newly initiated, brisk, low-level activity. In other words, when the patient initiates a new brisk, low-level activity, such as walking and the like, the accelerometer in the activity sensor provides a good indication of the sudden increase in the level of activity of the patient which generally results in heightened metabolic demand requiring the heart to deliver more oxygenated blood.
While activity sensors of this type are good at providing an indication of the onset of brisk, low-level activity, these sensors do have several shortcomings. For example, the signal that is often provided by such activity sensors becomes blunted when the patient is engaged in high exertion exercise. In other words, when the patient is heavily engaged in a particular physical activity, the activity signal may not provide a sufficient indication to the control unit of the need for more oxygenated blood as a result of the increased activity. For example, the output signal from a typical prior art activity signal is generally inaccurate for assessing the patient's actual metabolic need when the patient is performing an action like carrying a heavy object. The degree of acceleration detected by the activity sensor is likely to correspond to a perceived low metabolic demand activity, such as walking, and would not account for the increase in metabolic demand as a result of carrying the heavy object. Moreover, acceleration-based activity sensors are also subject to providing false readings as a result of the patient experiencing accelerations that are unrelated to physical activity, such as, for example, the patient travelling on a bumpy road in a vehicle.
Another type of sensor that is used to provide an indication of metabolic demand is referred to as a metabolic rate sensor. One common type of metabolic rate sensor is a minute ventilation sensor which measures the respiration rate and tidal volume of the patient's respiration. It is believe that the rate at which the patient is breathing and the volume of air being breathed is indicative of the metabolic demand of the patient. One typical way of obtaining a minute ventilation signal is to periodically measure the transthoracic impedance between a lead implanted within the patient's heart and an indifferent electrode, such as the housing of the implanted pacemaker control unit. As the transthoracic impedance is proportional to the chest volume, measuring this particular impedance value provides an indication as to the degree to which the patient's chest is expanding and contracting and the rate at which such expansion and contraction is occurring. The greater the patient's breathing rate and the greater the tidal volume of the breaths, the more likely it is that the patient has a heightened need for delivery of oxygenated blood by the heart.
While metabolic rate sensors, such as minute ventilation sensors, provide a strong indication of the metabolic demand of the patient, these sensors also have several disadvantages for use in determining the pacing rate and desired heart rate. In particular, the values provided by these sensors often lag in time behind the actual metabolic demand of the patient. Consequently, these sensors are typically not particularly well suited for providing the sole indication of the actual metabolic demand of the patient when the patient is initiating or ceasing physical exertion.
To address the problems associated with both of these types of sensors, rate responsive pacing systems have been developed which utilize the signals from several different types of sensors to determine a desired pacing and heart rate. For example, U.S. Pat. No. 5,626,622 combines the signals from an activity sensor and a minute ventilation sensor such that when the physical activity undergoes a transition, the combined response is predominantly derived by the physical activity sensor. Hence, the pacing rate corresponds to a pacing rate that is expected to satisfy the metabolic need for the observed level of activity as indicated by the activity sensor. During rest and steady state periods and rest or sustained exercise periods, the combined response of both the sensors, which determines the pacing rate, is predominantly derived from the minute ventilation sensor. Similarly, U.S. Pat. No. 5,562,711 is structured in such a manner that the activity sensor has an influence at lower pacing rates, such as when the patient is at rest. Various weighting factors are used so that when the patient is not at rest, a minute ventilation sensor provides the signal which is used to determine the desired heart rate and pacing rate. With both of the algorithms disclosed in these patents, the determination of the desired heart rate often involves comparatively complex algorithms which are expensive to implement and difficult to evaluate for diagnostic purposes. Further, these algorithms often fail to provide an optimum indication of a desired heart rate during recovery from physical exercise. Moreover, these algorithms are generally not capable of providing a desired heart rate suitable for a sleeping or rest mode of the patient.
Hence, there is a need for a pacing system and method which is capable of determining a desired heart rate based upon the inputs from an activity sensor and a metabolic rate sensor, such as a minute ventilation sensor. To this end, there is a need for a pacing system which is capable of simply and effectively determining the desired heart rate based upon the inputs from an activity sensor and a metabolic rate sensor such that the desired rate is clearly determined from the inputs of the two sensors for diagnostic purposes and also such that the desired rate has better response during periods of recovery from exercise of the patient.